Sleeping bags have long been used by hikers, campers, and other individuals in need of compact, portable bedding material. A first type of traditional sleeping bag has been of a generally rectangular configuration, and has included a bottom layer and a top layer folded over to form a common edge along one longitudinal side. A zipper is included along the edge to selectively close the longitudinal side and bottom end of the bag. The top edge was usually left open, allowing a users head to protrude.
One desirable characteristic of this above discussed traditional bag, is that this design often allowed two of these bags to be unzipped, unfolded, stacked and then re-zipped together, forming a two person sleeping bag with a single expanded sleeping compartment of uniform width and length. This dual sleeping bag arrangement was often found to be advantageous for couples, as well as allowing for shared body heat between the two individuals.
The majority of more recently produced conventional sleeping bags however, are of what is referred to as a "mummy bag" type design. This design has a shape more fitted to the human body, with an wider torso region that tapers to a narrower leg and foot region. The mummy bag configuration is designed to remove excess space within the bag so that the interior area which has to be heated by the body of the occupant is reduced to a minimum. This design also minimizes sleeping bag weight. This type of sleeping bag design also usually incorporates a zipper extending the length thereof, or that more typically extends only approximately 3/4 of the longitudinal length of the bag. Such bags can be zippered together in side by side fashion, to form a common upper torso area but separate leg and foot areas for two users. This design may not be satisfactory for some couples, and does not result in any weight or pack volume reductions for two hikers.